A split-birefringent coating technique, coupled with high-speed photography, was used to record the dynamic isochromatic fringe pattern associated with crack propagation and arrest in 4340 steel compact crack arrest fracture specimens. From the data contained in the isochromatic fringe patterns, the associated dynamic stress-intensity factor was calculated in two different ways. First, the dynamic stress-intensity factor was calculated from the isochromatic fringe information directly. Next, the isochromatic fringe patterns were used to obtain a crack position history which was used for input to a two-dimensional dynamic finite-element code used in the “generation mode.” These two dynamic stress-intensity factors were compared with each other and with the crack arrest toughness as calculated by the proposed ASTM method for estimating crack arrest toughness.